This invention relates to voltage regulators useful in battery charging system applications, and more particularly relates to techniques for regulating voltage in an automotive charging system.
In general, automotive charging systems include an alternating current generator, generally referred to as an alternator, that is operated by the engine from a drive belt and supplies a charging current to a voltage regulator, and a battery current and a constant output voltage to a battery and an automobile's electrical system. As the engine runs, the belt rotates causing the alternator through its stationary or stator windings to supply the charging current to the voltage regulator. The voltage regulator then regulates the alternators output voltage.
An alternator usually has a three-phase winding on the stationary or stator pole. A typical automotive alternator is composed of a shaft-mounted rotating field winding that is supplied field current. The field winding is wound on an inter-digitated pole. The inter-digitated pole is situated within a stationary pole. This stationary pole has multiple slots for receiving the stationary windings. An electric excitation current is applied as a feedback from the battery across the field winding. Rotating the inter-digitated pole as excitation current is applied across the field winding produces an alternating current in the windings of the stationary pole. This alternating current is then applied as a charging current to the input of the voltage regulator, and is applied through a three-phase bridge-rectifier as a battery current to the battery.
Conventional alternators employ a field winding, and to operate in the intended manner, require an external excitation current to the field winding. In the prior art, it is known to provide an excitation current to the alternator's field winding as a feedback current from the voltage regulator in response to changes in charging and current demands of the automobile's battery and electrical system. The alternator, in turn, responds to the excitation current across its field winding by increasing and decreasing the output charging current relative to the excitation current.
In operation, the magnitude of the direct current generated is dependent primarily on the field excitation current and the shaft's rotational velocity. To compensate for load demands and rotational accelerations, the electrical system voltage regulator modifies the field excitation current to maintain the alternator's output voltage at a constant level.
These basic fundamentals are integral to the operation of all prevalent systems in application. Primary differences which distinguish the various types of systems found in the industry relate directly to the field excitation source and activation thereof.
At low shaft rotational velocities there is a point in the operating characteristic of an alternator where it is not capable of producing battery current. This rotational velocity point is generally referred to as the cut-in rotational velocity. If excitation is supplied by the battery when the shaft's rotational velocity is lower than cut-in, unnecessary current drain will result.
As is known, this problem is readily solved by providing an excitation current from the alternator's stationary windings through a second set of three-phase bridge-rectifiers. In the simplest form the rectifiers are rated for the excitation current only. Below the cut-in rotational velocity the second set of bridge-rectifiers starves the stationary windings but reverse biases the first set of bridge-rectifiers resulting in no current drain to the battery. This ideal, self-excited technique relies heavily on residual magnetism in the rotor poles for activation. Hence, the alternator's cut-in rotational velocity is related to the rotor material's permeability and excitation current. Also, rotational velocity at which the stationary windings provide sufficient charging current for detection of shaft rotating, i.e. activation rotational velocity is related to the rotor material's coercive force. In order to allow the cut-in rotational velocity and the activation rotational velocity to coincide, stringent constraints must be imposed upon the rotor material's magnetic properties.
To overcome these constraints, the industry has evolved techniques employing electronic circuits to circumvent alternator performance limitations in regards to activation and excitation. These techniques allow more freedom to improve operational performance characteristics, and include coupling a current sensing resistor to a stationary winding and the base of a silicon
transistor. The transistor's collector then enables the voltage regulation circuitry to excite the alternator.
A drawback to this technique is that silicon transistors have typically a base emitter turn on voltage above 0.7 V. Consequently, the voltage regulation current does not activate the alternator until the alternator's shaft rotational velocity reaches 2,800 revolutions per minute. This delayed activation may result in increased battery drain.
Voltage regulators typically drive the field winding using power transistors that have limitations of three to eight amps of current. During operation these power transistors are switched on and off by sensing the field winding current and stationary winding current to regulate the battery voltage. Voltage regulators typically employ a feedback circuit to sense the current being fed through the field winding. This feedback circuit is necessary to assure that the power transistor transitions from an "off" state to an "on" state.
When the power transistor switches from an "off" to an "on" state using feedback circuitry, it may gradually cycle through a transition state. In this transition state, the power transistor tends to increase its power dissipation. This increase of power dissipation, in addition to wasting energy, may increase the operating temperature of the power transistor thereby decreasing the power transistor's life expectancy.
During operation of the alternator, surges in the voltage level occur across the voltage regulator. During these high voltage surges, generally referred to as a load dump condition, the power transistor may be switched "on." This may result in the voltage between the collector and emitter of the power transistor exceeding its operating parameters resulting in destruction of the power transistor and other voltage regulator components.
Voltage regulators are subjected to wide variations of temperature during operation. Operating characteristics of the voltage regulator's electrical components change with temperature variations. For the voltage regulator to maintain a constant level output, temperature compensating components, such as thermistors, must be included in the voltage regulator circuitry. However, thermistors are expensive as additional supporting electronics must be incorporated into the voltage regulator circuitry.